1. Field of the Invention
The present invention relates generally to a transparent display apparatus, and more particularly, to a transparent display apparatus constructed for selectively transmitting or blocking light of an image according to a supply of electric power to a conventional transparent organic light emitting diode.
2. Description of the Related Art
In general, an organic light emitting display device has a stack structure of an Organic Light Emitting Diode (OLED) and a Thin Film Transistor TFT layer. The OLED that emits light is stacked on the TFT layer forming an electric circuit, so that an emitting layer of the OLED can selectively emit light based on a signal controlled through the TFT layer.
The organic light emitting display device can be divided into an OLED display apparatus on a non-transparent substrate and a transparent OLED display apparatus on a transparent substrate. The structure of the transparent OLED display apparatus is described below.
The display apparatus includes a TFT layer (not shown) having a substrate, a gate electrode, a source region, a drain region, a via hole, etc.
As shown in FIGS. 1 and 2, the Transparent Organic Light Emitting Diode (TOLED) 1 includes a transparent anode 3, a hole transport layer 4, an emitting layer 5, an electron transport layer 6, and a transparent cathode 7, which are sequentially stacked on a glass substrate 2 of the TFT layer.
When a voltage is applied to the gate electrode, a channel in an organic semiconductor region is opened and a current flows from the source region to the drain region, which then passes through the transparent anode 3, the hole transport layer 4, the emitting layer 5, the electron transport layer 6, and the transparent cathode 7 of the TOLED 1 through the via hole. A light emitting function (that is, a function resulting from the application of an electric current) occurs in the emitting layer 5, when holes and electrons are combined and excited and the resulting energy is thus emitted in the form of light. This light refers to, for example, an emitting point corresponding to one pixel of a display panel.
As shown in FIGS. 1 and 2, the light emitting type can be divided into a downward emitting type in which light is emitted in a substrate direction, and an upward emitting type in which light is emitted in the opposite direction with respect to a light-emitting direction. Current trends favor the upward emitting type, in which the light can be emitted over a relatively wider region. The light in the upward emitting type can be projected out of a wide region through a black-matrix (not shown). The light in the downward emitting type can be projected out of only a region corresponding to the gate electrode. The cathode should be naturally transparent so that the light can travel in the upper direction. To this end, the conventional cathode was made to have a structure where indium-tin oxide (ITO) and silver (Ag) are stacked one by one.
A metal such as silver (Ag) is thinly laid and a transparent ITO is stacked on the metal in a desirable thickness. Thus, the conductivity characteristic of the metal and the light transmittance characteristic of ITO are combined. Here, the silver (Ag) is defined to have a thickness of about 5 nm, and the ITO is defined to have a thickness of about 50 to 200 nm.
Characteristics of the cathode should include good light transmission and the maintaining of a certain level of conductivity to prevent an unduly power consumption. Therefore, the cathode includes the ITO and the silver (that is, a metal) stacked on each other, which have the two above-mentioned characteristics.
The conventional TOLED may show its back side simultaneously, and such a characteristic may be considered as an advantage of the conventional TOLED. However, such a characteristic may sometimes decrease visibility or serve as a disturbance because the background can be seen.
When the conventional TOLED is applied to a watch-type mobile terminal, the watch-type mobile terminal always needs to display the time for a phone mode even when not in a communication state. However, when a display for the phone mode is adorned like that of a watch, it is necessary to keep turning on the display screen, which increases the power consumption of the mobile terminal. Moreover, it is impossible to adorn the watch-type mobile terminal with jewels as for an ordinary watch.